The work of Gunther Will, disclosed in U.S. Pat. No. 3,256,219, appears to represent the first successful attempt, coupled with an enabling disclosure, of the production of thermoset water-in-oil emulsions. Said patent and others disclose thermoset water-in-oil emulsions prepared with a variety of different oil phases, including those containing a mixture of an unsaturated polyester and a copolymerizable solvent. However, at the time the present invention was made, the art of producing polyester-containing thermoset water-in-oil emulsions was in a crude stage in its development. Widespread commercialization was hampered by the need for basic improvements not only in respect to such product characteristics as color and cell fineness, but also in respect to certain at least partially interrelated production characteristics such as emulsion stability, gel time, water exudation in the mold and cure shrinkage.
In accordance with the invention, it has been found that thermoset water-in-oil emulsions with superior properties and production characteristics result from the combined use of polyesters with molecular weights significantly above those of the average general purpose casting resins, and a particular promoter system. The polyesters used in accordance with this invention are obtained either by continuing the polyester producing polyesterification reaction for an extended period of time in order to increase the mean molecular weight to at least 1800, such as 1800 to 100,000, more preferably 2200 to 50,000, and most preferably 2800 to 30,000, or by terminating the polyesterification reaction when the polyester has reached a molecular weight such as 500 to 1800 and then modifying it with an amount of a coupling agent sufficient to increase the molecular weight to the desired value.
The mean molecular weight of the unmodified and modified polyesters useful in the present invention can be determined by conventional means such as solution viscosity, but is preferably determined by calculation from the reaction number, which is the sum of the acid value or acid number and the hydroxyl value, according to the following formula: EQU MW = 2(56,100)/RN
wherein:
Mw = mean molecular weight PA1 Rn = reaction number
Thus, from the above formula, it can be seen that the preferred polyesters of the present invention will have reaction numbers less than 62.5, such as 1.1 to 62.5, more preferably 2.2 to 51.0, and most preferably 3.7 to 40.1.
As used herein "acid number" is the number of milligrams of potassium hydroxide necessary to neutralize the acidity of one gram of the non-volatile content of a sample. The "hydroxyl value" is the number of milligrams of potassium hydroxide necessary to neutralize the acidity released by the reaction product of one gram (non-volatile basis) of the polyester and acetic anhydride. As those skilled in the art will readily appreciate, the hydroxyl vaue indicated by the titration results should be corrected for the acid value the polyester had prior to reaction with the anhydride--to obtain a corrected hydroxyl value. It is this corrected value which is used in calculating reactivity number and molecular weight herein. The non-volatile content of the material is determined by placing a one-gram sample of the material in a small aluminum pan on a hotplate at 300.degree. F for one-half hour at atmospheric pressure. When needed to prevent polymerization of the solvent, e.g. styrene, during the test, sufficient amounts of addition polymerization inhibitor(s) such as hydroquinone (see below) should be added to the sample prior to heating. The weight of the residue in grams multiplied by 100 equals the non-volatile content in percent.
The polyesters useful in the present invention are produced by reacting a polycarboxylic acid with a polyhydric alcohol at esterification temperatures, and generally between 150.degree. and 250.degree. C, until the acid value and the hydroxyl value of the reaction mixture have been reduced to values such that their sum equals the desired reaction number. These polyesters can be random polyesters produced by simultaneous addition of total quantities of all reactants or block polyesters produced by sequential addition of one or more reactants such as the saturated and unsaturated polycarboxylic acids.
Polycarboxylic acids which can be employed to produce the unsaturated polyesters useful in the present invention are both the saturated and unsaturated acids of 4 to 18 and preferably 5 to 8 carbon atoms. Examples of suitable saturated acids include, among others, oxalic acid, malonic acid, adipic acid, succinic acid, glutaric acid, sebacic acid, azelaic acid, as well as phthalic, isophthalic, and terephthalic acids which are preferred because they impart desirable physical characteristics such as compressive and tensile strength and impact resistance to the cured emulsion. Examples of suitable unsaturated acids include fumaric acid, itaconic acid, and maleic acid. Halogenated acids such as tetrachlorophthalic acid, tetrabromophthalic acid, and chlorendic acid (1, 4, 5, 6, 7,7-hexachloro-5-norbornene-2, 3-dicarboxylic) can also be employed. As used herein, the term "acid" is meant to include anhydrides, which are preferentially employed in order to minimize the water produced by the polyesterification reaction. Higher polycarboxylic acids such as trimellitic anhydride can be employed in amounts up to five weight percent which do not materially alter the linear nature of the polyester. However, in a preferred embodiment, the polycarboxylic acids consist essentially of dicarboxylic acids. The unsaturated acid is employed in an amount sufficient to provide the resultant polyester with ethylenic unsaturation capable of reacting with the copolymerizable solvent. Generally, the molar ratio of unsaturated acid to saturated acid is from 1:0 to 1:5.
The polyhydric alcohols which can be reacted with the polycarboxylic acids in order to give polyesters useful in the present invention are preferably the dihydric alcohols, examples of which include, among others, ethylene glycol, diethylene glycol, 1,2-propylene glycol, 1,2- or 1,3-dipropylene glycol, 1,3-propylene glycol, 1,3-butylene glycol, 1,2-butylene glycol, neopentyl glycol, 1,3-pentanediol, and 1,5-pentanediol. Persons skilled in the polyester resin art will readily observe that equivalent polyesters can be formed from "glycol anhydrides", e.g. alkylene oxides such as for example ethylene propylene and butylene oxides, and the term "polyhydric alcohols" should be interpreted in light of this fact. Higher polyhydric alcohols such as trimethylol propane and pentaerythritol can be used in minor amounts up to 5 weight percent which do not materially alter the linear nature of the polyester, but in a preferred embodiment the polyhydric alcohol consists essentially of dihydric alcohols.
The molar ratio of polycarboxylic acid to polyhydric alcohol is generally from 10:8 to 8:10, although stoichiometric ratios or those having up to 10 to 20 mole percent excess polyhydric alcohol can also be employed. When operating outside these ranges, it is frequently impossible to reduce the reaction number to the desired value within a reasonable period of time.
In one aspect of the present invention, the high molecular weight polyesters are produced by reacting a polyester having a molecular weight of 500 to 1800, and preferably 800 to 1500, with a polyfunctional, preferably a difunctional, coupling agent capable of reacting with hydroxyl and/or carboxylic acid groups on the polyester. Examples of suitable polyfunctional coupling agents include, among others, phosgene, polyacid halides, alkoxy melamine formaldehyde condensate compounds, polyepoxides, anhydrides of polybasic organic carboxylic acids, and the polyisocyanates, which are preferred.
Suitable polyisocyanates are cyclic or aliphatic polyisocyanates having from 2 to 5, preferably 2, isocyanate groups. Less preferably, isocyanate prepolymers formed by reacting excess polyisocyanate with with polyhydric alcohol can be employed. Examples of suitable polyisocyanates include, among others, the aliphatic polyisocyanates such as hexamethylene diisocyanate; alicyclic polyisocyanates such as 4,4'-dicyclohexylmethane diisocyanate; and aromatic polyisocyanates such as 2,4-and 2,6-toluene diisocyanate; diphenyl methane diisocyanate and the dimethyl derivative thereof, as well as 1,5-naphthalene diisocyanate; triphenyl methane triisocyanate; xylylene diisocyanate, and the methyl derivative; polymethylene polyphenyl isocyanate; chlorophenylene-2,4-diisocyanate, and mixtures thereof. Toluene diisocyanate is preferred because of its cost, availability, reactivity, and relatively low toxicity.
Examples of suitable polyepoxides include the diepoxide of cyclopentadiene and Epon 828 available from the Shell Oil Company.
Among the suitable polyacid halides, preferably diacid chlorides, are adipoyl chloride, phthaloyl chloride, and succinoyl chloride. When using these diacid halides, the coupling reaction produces a hydrogen halide such as HCl which should be inactivated because of the undesirably low pH of the resultant mixture. Excessively low pH's adversely affect emulsion stability, and the resulting need for inactivation makes the polyacid halides somewhat less preferred coupling agents. The hydrogen halides can be inactivated by physical removal such as subjecting the coupled resin to subatmospheric pressures and preferably those below 30 mm of Hg in a suitable apparatus such as an aspirator, or more preferably by neutralization with an organic or inorganic base. Examples of suitable inorganic bases include sodium hydroxide, potassium hydroxide, their corresponding oxides, and calcium carbonate which is preferred. The organic bases can be ammonia or the mono-, di- or tri-lower alkyl amines such as triethylamine, trimethylamine, triethanolamine, as well as pyridine and quinoline which are preferred because of the insolubility of their hydrochloride salts which facilitate their removal by filtration. Of course, a combination of 2 or more of the above methods can be employed to inactivate the hydrogen halide. When neutralizing the resin with a base, the base can be added directly to the resin either before or after addition of the diacid halide or, alternatively, the base can be added to the water with which the resin is to be emulsified.
The coupling is effected by simply mixing the polyester and the coupling agent at temperatures above that at which the reaction mixture solidifies and below that at which degradation of the reactants takes place, generally between 0.degree. and 100.degree. C. The completion of the coupling reaction can be determined by cessation of increase of molecular weight or viscosity, or by titration for unreacted coupling agent such as polyisocyanate. When the coupling agent is diisocyanate, the coupling reaction proceeds according to the following equation: ##STR1## wherein R.sup.1 is H-- or HO--R.sup.2 --; R.sup.2 is lower alkylene or alkoxy lower alkylene; R.sup.3 is a divalent radical selected from the group consisting of phenylene, lower alkylene, and vinylene wherein at least a portion thereof is vinylene, m and n are integers equal to 1 to 8 inclusive; and R.sup.4 is a phenylene or lower alkylene. The weight ratio of coupling agent to polyester is generally 1:100 to 50:100, and preferably 3:100 to 10:100.
Solvents which are useful in the present invention are those which are copolymerizable with the ethylenically unsaturated polyester, and which will dissolve the polyester, examples of which include vinyl toluene, alpha methyl styrene, acrylonitrile, ethyl acrylate, methyl acrylate, methyl methacrylate, vinyl acetate, triallylcyanurate, diallyl phthalate, methyl vinyl ether, ethyl vinyl ether, and styrene, which is preferred because of cost, availability and reactivity. The copolymerizable solvent is employed with the unsaturated polyesters in weight ratios of 10:1 to 1:10 and preferably 1:2 to 2:1.
In a preferred embodiment of the present invention wherein the mixture of polyester and copolymerizable solvent exhibits exceptional shelf life, the mixture contains an addition polymerization inhibitor. Addition polymerization inhibitors which can be used to inhibit addition polymerization during the reaction of the polyol and the polycarboxylic acid, or which can be used to inhibit polymerization of the stored composition comprising the uncured, curable linear polymer and the ethylenically unsaturated compound include, among others, hydroquinone, benzaldehyde, ascorbic acid, resorcinol, and symmetrical di (.beta.-naphthyl)-p-phenylene diamine. These inhibitors can comprise up to 0.5 weight percent or more of the composition.
The present invention is based on the discovery that -- contrary to expectations based on manipulation of the prior art -- one can produce thermoset emulsions with a number of the above-mentioned desirable product and production characteristics when using, in a water-in-oil emulsion, a curing system composed of peroxide free radical generating catalyst, cobalt compound and tertiary aromatic amine, in conjunction with a polyester having a molecular weight in the range specified herein. As will be evident from the examples, which illustrate the invention, a solution is formed which includes the modified or umodified polyester, copolymerizable solvent and the cobalt compound. Prior to or during its formation this solution is mixed with water to form a water-in-oil emulsion. When completed, the emulsion also contains the tertiary aromatic amine promoter and peroxide catalyst. The cobalt compound in solution in the oil phase and the molecular weight characteristic of the polyester contribute jointly to the stability of the emulsion. The molecular weight also contributes with the tertiary aromatic amine to the rate at which the polymerizable water-in-oil emulsion will cure at a given level of peroxide catalyst and cobalt promoter.
The unique cooperation of these various factors was not apparent to leading workers in the relevant art. When Gunther Will disclosed the preparation of water-extended polyesters and other polymers in U.S. Pat. No. 3,256,219, he listed various "initiators and activators," including inorganic or organic cobalt salts and tertiary amines but directed his working examples to polyester having an acid number consistent with a lower molecular weight range than specified herein, cured without oil soluble cobalt salt. In view of the very substantially improved result provided by the present invention, he would have disclosed it had he recognized it. The fact that the invention was overlooked was perhaps a normal result of a technical prejudice among persons skilled in the polymer art due to their knowledge that water inhibits cobalt-promoted curing systems. Even those subsequent workers who presented working examples based on the use of cobalt compounds or tertiary amines in peroxide-catalyzed polymerizable water-in-oil emulsions (see U.S. Pat. No. 3,244,772) failed to grasp the discovery. They did not recognize that production characteristics and/or product properties could be improved when the above three-component curing system was used in conjunction with polyesters of specified molecular weight, whether the polyesters were modified or not. Thus, in order to provide desired emulsion stability and/or product characterists, certain of these workers turned to using polyesters with polyalkylene oxide tails, such as for instance as is disclosed in U.S. Pat. No. 3,442,842, apparently discarding the cobalt-peroxide curing systems.
In accordance with the invention, the oil phase is cured with the aid of a free-radical-forming peroxide catalyst. Any peroxide which is liquid or soluble in the emulsion and whose break-down into free radicals is promoted by cobalt is suitable, including for instance, hydrogen peroxide, methyl ethyl ketone peroxide, di-tert-butyl peroxide, tertbutyl hydroperoxide, isopropyl peroxidicarbonate, dichlorobenzoyl peroxide, lauroyl peroxide, acetyl peroxide, tert-butyl peracetate, tert-butyl perbenzoate, di-cumyl peroxide, di-ethyl peroxide, ditertamyl peroxide, and cyclohexyl hydroperoxide. The free radical-forming compound is employed in such concentration as is required to rapidly initiate the curing of the polyester and the vinyl monomer employed.
Suitable cobalt compounds which form solutions with the polyester and copolymerizable solvent include for instance cobalt naphthenate, cobalt octoate and their equivalents. Cobalt naphthenate and cobalt ocotoate are oil-soluble cobalt compounds which are known promoters for peroxide curing catalyst systems. The organic moieties of such cobalt compounds provide solubility in the polyester and solvent, but it is recognized in the art that the cobalt ion provides the reaction with the catalyst to promote release of free radicals for the reaction of a polyester and copolymerizable solvent. In this connection, see the SPI Handbook of Reinforced Plastics, Oleesky and Mohr, Reinhold, 1964, pp. 42-48. Thus, when this disclosure and the appended claims refer to cobalt octoate and/or cobalt naphthenate, such compounds will be understood to be representative of the entire class of organic cobalt compounds which will dissolve in the polyester and copolymerizable solvent, thus making cobalt ions available in the oil phase for promoting the curing thereof. Examples of other cobalt compounds falling within this class are cobalt neodecanate (which is preferred), cobalt linoleate, and cobalt salts of refined and unrefined organic acid mixtures.
Suitable tertiary amines include for instance dimethyl aniline (preferred) dimethyl paratoluidine and their equivalents. These compounds will be understood to be representative of the known class of tertiary aromatic amine promoters for peroxide curing catalyst systems. Other useful compounds in this class include, without limitation, N,n-diethyl aniline, phenyl diethanol amine, phenyl ethyl ethanol amine, N-ethyl-m-toluidine, meta toluidine and the like.
Guided by the examples of the invention given below, and their own general knowledge, persons of ordinary skill in the art of polyester-based thermosetting water-in-oil emulsions will readily select amounts of the peroxide catalyst, cobalt compound and tertiary aromatic amine which are sufficient for curing the emulsions. For instance, the concentration of peroxide catalyst needed is known to vary depending on the curing conditions and the particular polyester employed, but is generally within the range of 0.001 to 10 percent by weight based on the weight of the polymerizable solvent or vinyl monomer which is present. Generally, the optimum amount is considered to lie in the range of about 0.5 to about 2.5%.
The experimental evidence is such as to indicate that the cobalt and/or cobalt compound contributes appreciably to the stability of an emulsion in accordance with the invention, even where present in an amount which fails to cure the emulsion within the short cure times demanded by typical mass production casting processes. In experiments with the invention wherein low and progressively higher levels of cobalt were used, it was found that before significant promotion was obtained, a certain minimum level of cobalt had to be supplied. The portion of the cobalt supplied up to this minimum evidently is held by, or reacts, complexes, or otherwise cooperates with the polyester and/or the water in a way which significantly fosters stability in the emulsion, but does not appear to be available for promotion. That some reaction with the polyester is involved is suggested by the fact that this minimum level seems to vary with the acid value of the polyester. For example, for one polyester, WEP 11*, having a molecular weight of 3700 by the above reaction number formula, this minimum is approximately 0.3 parts of a 12% metal solution per 100 parts polyester and styrene, whereas for another polyester, WEP 27*, having a similarly determined molecular weight of 2800, this minimum is 0.5% (* - WEP 11 and WEP 27 are products of Ashland Oil, Inc.). Interestingly, amounts below these minima will, under typical circumstances, vigorously promote the cure of polyesters in non-aqueous systems; and those skilled in the art will readily note from the examples below that the invention employs substantially more cobalt than is employed in most circumstances with polyesters in non-aqueous systems. Such additional cobalt as is present above the minimum level is apparently available for the normal peroxide promotion reaction which is believed to involve alternate donation and acceptance of electrons as the cobalt repetively changes its valence state from Co.sup.+2 to Co.sup.+3 to Co.sup.+2 and so forth. It should therefore be apparent that when one practices the invention with amounts of cobalt effective to promote curing, he will also have inherently supplied the quantity of cobalt required to procure the emulsion stabilization benefits discussed above. Optimum levels of cobalt (12% solution basis) are considered to lie in the range of about one to two percent for preferred polyesters of molecular weight in the range of about 2500 to about 4000.
Apparently, the cobalt which is present above and beyond the minimum level discussed above would be substantially inactivated in the absence of the tertiary amine. Available evidence suggests that water reacts more rapidly with Co.sup.+3 than the peroxide, thus interrupting the cobalt in its function of helping break down the peroxide, unless the tertiary amine is present to stabilize the cobalt through complexation. Other things being equal, the optimum amount of tertiary aromatic amine promoter is generally considered by those skilled in the art to increase or decrease in accordance with increases or decreases in the number of cobalt ions available for complexation therewith. The relationship is not linear (perhaps due to a portion of the available cobalt ions having reacted with the acid groups of the polyester) but the optimum weight of for example dimethyl aniline will generally be about one third that of the cobalt (as shown by the examples) and will lie in the range of about 0.25 to about 1.25% for the preferred polyesters of a molecular weight of about 2500 to about 4000.
In any event, those skilled in the art are experienced in determining the optimum amounts of catalysts and promoters for a given set of curing conditions by simple experiments in which the invention is carried out repetitively under the given conditions with planned variations in the amounts and ratios of tertiary amine, cobalt compound and catalyst, while noting the gel time, gel to peak exotherm and peak exotherm of the curing reaction and observing the physical properties of the product. The results are then correlated in a known manner with the amounts used. Generally, the total amount of promoters will be an effective amount within the range of 0.1 to 5% by weight of the oil phase.
The emulsions of the present invention can be prepared by simply mixing the copolymerizable solvent and the modified or unmodified polyester with water by any suitable means such as an egg-beater or an air-driven stirrer, or by use of centrifugal pumps at weight ratios of 1:10 to 10:1 and preferably 1:4 to 2:1 of polyester and copolymerizable solvent to water. According to the various preferred methods, the various emulsion components, certain of them being premixed, are placed in various reservoirs. Each of the reservoirs is equipped with a pump at its outlet, and the outlet stream from each is metered into a mixing chamber (having a relatively high shear mixing means therein) wherein the streams are emulsified. The mixing chamber outlet is equipped with a delivery tube which can be directed towards a location where the emulsion is to be cured. In view of the fact that the preferred method of preparation discharges the emulsion through a delivery tube by a pumping action arising out of the operation of the aforesaid pumping means and/or mixing means, it is preferred that a pumpable (and most preferably a pourable) emulsion be formed. Those skilled in the art will appreciate that the pourability and pumpability of an emulsion of the present type are basically a function of emulsion viscosity, and that the latter depends mainly upon such factors as the ratios of oil phase to water phase, the ratio of solvent to polyester, the molecular weight of the polyester, water droplet size, the presence of various additives, temperature and the like. Within the context of stable emulsions, lower viscosities are obtained through the use of higher ratios of oil phase to water phase and solvent to polyester, lower molecular weight polyesters within the above-described ranges, and using the emulsions at higher temperatures. Reverse manipulation of these factors can be used to increase viscosity. According to the method of emulsion preparation which was formerly particularly preferred, the polyester, vinyl monomer solvent and catalyst are premixed to the desired levels and placed in a reservoir. In a second reservoir is placed water, and in a third reservoir is placed promoter. Each of the reservoirs is equipped with a pump at its outlet, and the outlet stream from each is metered into a mixing chamber (having a relatively high shear mixer) wherein the three streams are emulsified.
According to the presently preferred method of forming the emulsion, the polyester, vinyl monomer solvent, the cobalt compound promotor and the tertiary amine promoter are premixed to the desired levels and placed in a reservoir. In a second reservoir is placed water and water-soluble or -dispersable peroxide catalyst at the desired level. Each of the two reservoirs is equipped with a pump at its outlet, and the outlet stream for each is metered into a mixing apparatus such as is disclosed by Clocker in U.S. Pat. No. 3,679,182.
The natural flame retardancy of the water-containing thermoset emulsions of the present invention can be enhanced according to know procedures such as by the addition of antimony trioxide and/or the use of halogenated reagents such as the above-described chlorinated and brominated polycarboxylic acids.
The polymerized emulsions of the present invention find utility as materials of contruction, thermal insulation, and filters and have other uses as described in U.S. Pat. No. 3,256,219.
In the following examples, all parts are by weight unless the contrary is clearly indicated.
When preparation of an emulsion is attempted, the emulsifying technique includes holding the oil phase under vigorous agitation with a propellor mixer in a paper beaker, gradually adding the water phase as a fine stream at the vortex which is formed in the oil phase by the mixer, endeavoring to add aqueous phase only as fast as it disappears, and --as necessary-- using a wooden paddle to help (or in the attempt to help) incorporate any slight localized excesses of water which appear at the surface of the resultant emulsion, dispersion or mixture around the periphery.